I work at a football club and I need to install some long 20 ft metal poles into the ground set in concrete. My question is how deep must the holes be and what diameter hole needs digging. The poles are ex scaffold poles and are 3 inches in diameter. The poles are being installed to hold up netting behind the goals to stop the ball from going into adjoining fields and roads etc. Thanks for any help
The primary stress in this case will be wind. Typically you'd embed a pole at a ratio of 1:4. In your case, if the poles are 20' above ground, you'd want to be 4-5 feet down. If your poles are 20' in length (16-17' above ground), 3-4' may be adequate.
I'd probably use form tubes (or cleanly-dug holes) of a 10 or 12" nominal diameter (at least 3 time the pole diameter). You wouldn't want less than 3-4" of concrete outside the pipe to prevent breakout.